DESCRIPTION (adapted from the application) Primary biliary cirrhosis is a chronic autoimmune liver disease characterized by inflammatory destruction of small and medium sized intrahepatic bile ducts, leading to cholestasis, then fibrosis, cirrhosis, and its complications. Although the etiology is unknown, T cells that surround the bile ductules are the likely mediators of bile duct destruction. Other cell types within the portal infiltrate, including macrophages and B cells, probably contribute to this autoimmune process. However, the manner in which these cells communicate and collaborate to cause bile duct lesions is not understood. The hypothesis of this proposed project is that T cells orchestrate the immune attack on cholangiocytes via signaling to other cells in the liver through CD40 ligand. Specifically, expression of CD40 ligand by activated T cells in the liver stimulates B cells to increase production of immunoglobulins; activates macrophages to produce IL-12, which leads to a shift of T helper cells to the Th1 phenotype and subsequent IL-2 and IFN-gamma production; and induces cholangiocytes to undergo apoptosis at an increased rate. This hypothesis will be tested by a correlative analysis of the level of expression of CD40 ligand in the liver of PBC patients with the potential consequences of CD40-CD40 ligand interaction; such as upregulation of immunoglobulin and cytokine production, and apoptosis of cholangiocytes. Expression of CD40 and CD40 ligand will be localized in the liver with immunohistochernistry. These patient-oriented studies will be supplemented by in vitro experiments designed to study CD40-ligand induced apoptosis in cholangiocytes. These studies will provide insight into the role of T cells in the pathogenesis of PBC and provide information useful in developing new treatment modalities.